A Fischer-Tropsch reaction generally entails contacting a stream of synthesis gas with a catalyst under temperature and pressure conditions that allow the synthesis gas to react and form hydrocarbons. More specifically, the Fischer-Tropsch reaction is the catalytic hydrogenation of carbon monoxide to produce any of a variety of products ranging from methane to higher hydrocarbons and aliphatic alcohols. Research continues on the development of more efficient Fischer-Tropsch catalyst systems and reaction systems that increase the selectivity for high-value hydrocarbons in the Fischer-Tropsch product stream.
Originally, the Fischer-Tropsch synthesis was operated in packed bed reactors. These reactors have several drawbacks, such as temperature control, that can be overcome by gas-agitated slurry reactors or slurry bubble column reactors. Gas-agitated reactors, sometimes called “slurry reactors” or “slurry bubble columns,” operate by suspending catalytic particles in liquid and feeding gas reactants into the bottom of the reactor through a gas distributor, which produces small gas bubbles. As the gas bubbles rise through the reactor, the reactants are absorbed into the liquid and diffuse to the catalyst where, depending on the catalyst system, they are typically converted to gaseous and liquid products. As the gaseous products are formed, they enter the gas bubbles and are collected at the top of the reactor.
Because of the formation of liquid products (commonly called waxes), the slurry needs to be maintained at a constant level by continuously or intermittently removing wax from the reactor. The problem with wax removal is that catalyst in the wax must be separated from the slurry and returned to the reactor to maintain a constant inventory of catalyst in the reactor. Several means have been proposed for separating the catalyst from the wax, e.g., centrifuges, sintered metal filters, cross-flow filters, woven-wire mesh, magnetic separators, gravitational settling, etc.
The separation task is most challenging when the catalyst particles break down during operation to produce “fines” which could be as small as sub-micron in size. Independent of the catalyst-wax separation systems being used (i.e. centrifugation, settling, filtration, hydrocyclones, or magnetic separation), the presence of ultra-fine particles decreases the efficiency of the separation system.
Some of the early work on catalyst/wax separation by placing filter on an external slurry circulation loop is described in an article by M. D. Schlesinger, J. H. Crowell, Max Leva and H. H. Storch titled “Fischer-Tropsch Synthesis in Slurry Phase” from the U.S. Bureau of Mines (Engineering and Process Development, Vol. 43, No. 6, page 1474 to 1479, June 1951).
When a cake is allowed to form on a substrate, its continuous growth will result in a lower filtrate flux unless continuous backwash cycles are performed, therefore lowering the overall efficiency of the filtration system. To partially overcome this limitation the separation systems are over designed in order to account for this loss of efficiency. Thus, there remains a need in the art for methods and apparatus to improve the removal of wax products from a slurry with a high solids content, such as a Fischer-Tropsch slurry. Therefore, the embodiments of the present invention are directed to methods and apparatus for filtering a slurry that seek to overcome these and other limitations of the prior art.